1. Field of the Invention
The present invention relates to a booster circuit that is used in a power supply circuit, a power supply device, or the like, and is capable of outputting a boosted input voltage.
2. Description of the Related Art
As disclosed in Japanese Laid-open Patent Application No. 2005-224058, the applicant of the present invention has proposed a booster DC/DC converter wherein even a small-sized converter can handle a large amount of electricity by preventing magnetic saturation in a core of the converter by magnetic offset. Specifically, the booster DC/DC converter includes a transformer having the first and the second coils wound around the same core in reverse direction (opposite phase), and a pair of switches. An exciting current flows through the first coil, and at the same time a current is generated in the second coil in such a direction that magnetic flux of the core is offset. Then, the current in the second coil is fed into an output side via a diode, thereby obtaining an output voltage, which is approximately twice as high as the input voltage.
Another example of a booster circuit is shown in FIG. 12. FIG. 12 illustrates an electric diagram of a configuration of a conventional booster circuit. A booster circuit 9 has a classical circuit configuration, which is a prototype of various types of booster DC/DC converters. The booster circuit 9 includes a capacitor C11 interposed between an input terminal Vin and a common terminal COMMON, an inductor L11 provided on an input side and having one end connected to the input terminal Vin, a diode D11 having its anode connected to another end of the inductor L11 and its cathode connected to an output terminal Vout, a switch element SW11 interposed between the common terminal COMMON and a connection point of the inductor L11 and the diode D11, and a capacitor C12 interposed between the output terminal Vout and the common terminal COMMON.
Furthermore, there has been proposed a direct-current power supply device for obtaining an output voltage, which is more than twice as high as the input voltage, as disclosed, for example, in Japanese Laid-open Patent Application H11-146635. FIG. 13 illustrates an electric diagram of a booster circuit disclosed in the application. In the booster circuit, a winding wound around a transformer T15 is provided with a center tap P13, and taps P12, P14, which are arranged symmetrically about the center tap P13. One input voltage is applied to the center tap P13, and the other input voltage is applied to the taps P12, P14 via switch elements SW15, SW16, so that an output voltage is obtained from both ends of the winding via diodes D15, D16 by a wired OR technique.
However, the booster DC/DC converter disclosed in the above-mentioned Japanese Laid-open Patent Application No. 2005-224058, which employs a principle of converting a voltage by a converter, is required to have a higher voltage boost ratio.
Because the reactor (inductor) L11 itself performs a boost function in principle in the booster circuit shown in FIG. 12, the large reactor L11 is required and the output smoothing capacitor C12 is required to have a large capacity so as to withstand a large ripple current. Therefore, it is difficult to reduce the whole size of the booster circuit shown in FIG. 12.
In the direct-current power supply device disclosed in the above-mentioned Japanese Laid-open Patent Application H11-146635, for example, when the switch element SW15 is turned on, a raised voltage is induced in a terminal P15 of the transformer T15, and at the same time a negative voltage occurs in a terminal P11 of the transformer T15. As a result, inverse voltage is applied to the diode D15 (when the switch element SW16 is turned on, an inverse voltage is applied to the diode D16), and therefore the diodes D15, D16 need to have a higher inverse voltage withstand. Furthermore, a current does not flow in the negative voltage in the terminal P11, thereby easily causing a surge voltage.
In the booster chopper circuit, which employs a transformation function of a transformer as described above, a large core is required in order to prevent magnetic saturation upon direct-current magnetization, thereby making it difficult to reduce the size and the weight of the circuit.